Derivates of streptokinase which are able to convert plasminogen to plasmin in varying degrees in different species will be isolated and chemically characterized to gain insight at a molecular level into the reaction between streptokinase and plasminogen. The derivatives will be tested for fibrinolytic potency. A new caseinolytic assay for plasminogen proactivator and activator in the blood will be used in biochemical, physiological and clinical studies. An attempt will be made to determine the site(s) of production of the proactivator.